Such viewers are known, in particular from U.S. Pat. Nos. 4,726,670 and 4,892,399. These viewers are superior to earlier spyhole devices inasmuch as both have optics that produce real, projectable images which can be observed from some distance, as distinct from the earlier devices that produced virtual images which required the user's eye to be close to the eyepiece.
Yet both the the above prior-art viewers suffer from disadvantages that reduce their usefulness. Both devices, and in particular the device described in U.S. Patent. Nos. 4,892,399, require large holes to be provided in the door, in the case of the latter, mainly due to the use of a double Dove prism 9, 9' as erector element, even a stepped, two-diameter hole very difficult to produce and certainly unsuitable for do-it-yourself installation, with retrofitting, starting out from an existing, relatively small bore being a very tricky business even for an expert.
Apart from the above difficulties, such a large opening also constitutes a definite danger as, by knocking out the viewer, access may be gained for tampering with the door lock from the inside of the door.
The viewer according to U.S. Pat. No. 4,726,670 has several more disadvantages. Its optical system is clearly not a wide-angle system, very important for this sort of application, and in both disclosures the efficiency of their ground-glass screen is severely compromised by its being exposed to ambient light, which greatly reduces contrast.
A further drawback is common to both disclosures: the orientation of the optical axis is fixed. For closer viewing, this forces taller users to stoop, while users of smaller stature may have to stand on their toes and even when viewing the screen from a larger distance, truly satisfactory results are obtained only when the diffusing screen is viewed in a direction more or less perpendicular thereto.